The field of the present invention relates generally to microdevices and microstructures, and more particularly to a microfabrication process which enables the creation of a millimeter-scale, large area movable structure integral with and supported by, micron-scale micromechanical flexures, actuators and/or transducers.
The term microelectromechanical systems (xe2x80x9cMEMSxe2x80x9d) refers to a new technology in which electrical and mechanical devices are fabricated at substantially microscopic dimensions utilizing techniques similar to those well known in the manufacture of integrated circuits. Such devices will be referred to herein as MEMS devices or micromechanical devices for convenience, although it will be understood that present commercial applications of MEMS technology include microelectromechanical transducers such as pressure sensors, inertial measurement devices, electrostatic actuators, and the like, as well as a wide variety of nanometer-scale micromechanical support structures. For an introduction to the use of MEMS technology for sensors and actuators, see for example the article by Bryzek et al. in IEEE Spectrum, May 1994, pp. 20-31.
The application of this technology to inertial measurement devices has received a great deal of attention from the microelectromechanical community, as evidenced by the paper by Kuehnel and Sherman xe2x80x9cA surface micromachined silicon accelerometer with on-chip detection circuitry,xe2x80x9d Sensors and Actuators A 45 (1994), pp. 7-16, and by U.S. Pat. Nos. 5,245,824, 5,563,343, 5,126,812 and 5,095,752. Microaccelerometers are available as commercial products, and most of these devices have been applied to the sensing required for deployment of airbags in automobiles. This application requires an accelerometer sensitive to accelerations in the range of 50 g (490 m/s2), and microaccelerometers offer size, cost and performance advantages over prior technologies, such as piezoelectric devices, for inertial sensing. There is, however, substantial interest it obtaining micromechanical accelerometers capable of sensing much smaller levels of acceleration, for example in the range of micro-g or even nano-g""s (10xe2x88x925 to 10xe2x88x928 m/s2), but these low ranges of acceleration have eluded MEMS devices due to the inherent requirement for larger masses to sense smaller accelerations. Although MEMS fabrication techniques are versatile, they are inherently limited as to the surface area, the size and the mass of structures that can be produced.
One attempt to overcome the mass limitations of MEMS structures in accelerometers has been the use of electron tunneling transducers to provide extremely sensitive measurements of the very small displacements resulting from low levels of acceleration. The paper by Rockstad et al., xe2x80x9cA miniature high-sensitivity broad-band accelerometer based on electron tunneling transducers,xe2x80x9d Sensors and Actuators A 43 (1994), pp. 107-114, discusses such a device, but the disadvantage of this approach is the complexity of the fabrication process required to obtain such an accelerometer. Furthermore, there are serious issues regarding the long term stability of the tunneling transducer, and accordingly such devices are not well suited to widespread commercial applications such as automotive and consumer products.
The use of wafer bonding techniques to create wafer-thick silicon structures which can serve as large masses, or the addition of layers of heavier materials such as gold as described in the paper by Roylance and Angell, xe2x80x9cA Batch-Fabricated Silicon Accelerometer,xe2x80x9d IEEE Trans. Electron DevicesED -26 (1979), pp. 1911-1917 have also been suggested. These approaches have the severe disadvantage of utilizing complex, and expensive, fabrication processes resulting in devices which are not competitive in the commercial marketplace. Therefore, it is desirable to find a cost-effective micromechanical fabrication technology, such as plasma micromachining, to fabricate improved high mass structures which can function as accelerometers in MEMS devices. What is needed is a novel approach to the design and manufacture of micromechanical accelerometers of arbitrary size and shape in which such high mass structures can be obtained to provide for high sensitivity accelerometers without the introduction of complex, low yield manufacturing steps.
The use of MEMS devices as actuators, is described, for example, in the papers by Hirano et al. xe2x80x9cDesign, Fabrication, and Operation of Submicron Gap Come Drive Microactuators,xe2x80x9d J. Microelectromechanical Sys. 1 (1992), pp. 52-59, and Jaecklin et al. xe2x80x9cComb actuators for xy-microstages,xe2x80x9d Sensors and Actuators A 39 (1993), pp. 83-89. Such actuators are used to effect switching functions, direct fluid flows, move valve assemblies, tilt mirrors, move microstages, and to carry out a wide range of other functions in various microstructures. However, these MEMS actuators have limited dimensions by reason of the process used to fabricate them, and there is a need for a reliable process for making large area structures for use in micromechanical devices. Such large area surfaces would have numerous applications in research as well as in commercial products such as high density data storage, optical deflectors, and the like. Thus, it is desirable to have large area, flat microstages which are capable of being controllably scanned along an axis or in two orthogonal directions (x and y). Moreover, it is necessary that such stages be capable of being scanned over relatively large distances, several tens of micrometers for example. Accordingly, there is a need for an effective process for fabricating large area, optically flat, micromechanical stages coupled with electrostatic actuators capable of large motion actuation in one or two directions.
What is required for both large mass accelerometers and large area microstages is a process for fabricating a large area structure having dimensions up to several millimeters, releasing that structure for motion, and integrating that structure with other micromechanical and microelectromechanical devices which may have dimensions in the range of 1-3 xcexcm. It is further desirable that all of the structures be fabricated from a single crystal silicon substrate material. Moreover, substantially the same fabrication process should be utilized for the creation of the large area structure and other micromechanical and microelectromechanical devices, although it should be understood that there may be circumstances under which it is more effective or economical to utilize a different fabrication process for creation of the large area structure. Further, what is needed is the ability to readily integrate the large area micromechanical devices with microelectronic circuits which may be located on the same wafer, for such circuits are required for signal conditioning, for control of the actuation of the large area structure, and for sensing its motion.
In order to achieve the foregoing and to overcome the problems inherent in fabricating large area released microstructures, the present invention is directed to a monolithic process for making silicon micromechanical devices in which large area, movable structures are integral with micromechanical flexible supports, or flexures, and microelectromechanical sensors and/or actuators.
A further aspect of the invention is a fabrication technique which permits the integration of a large area released structure with conventional micromechanical devices. The conventional micromechanical devices may be flexible supports, may be motion transducers (capacitive or otherwise) and/or electrostatic actuators, and may be comprised of released beam segments formed with substantially the same processing techniques, such as plasma micromachining, as the large area structure.
Another aspect of the invention is the provision of micron-scale, flexible silicon beam support members, or flexures, capable of supporting a large, millimeter-scale high mass structure and enabling its motion in a desired direction(s) while substantially precluding motion in any other direction. High aspect ratio microbeam structures are utilized in the present invention to support the structure and to provide a requisite mechanical stiffness to prevent out of plane motion, while permitting controlled in-plane motion of the structure.
A still further aspect of the invention is the use of silicon beam flexures such as folded springs to enable large distance electrostatic actuation of large mass structures without introducing mechanical instabilities in the moving structures.
A further aspect of the invention is the use of substantially the same plasma micromachining technique, involving lithography, deposition, and reactive ion etching, at different stages of the device fabrication, from both the front (or top) side of a substrate such as a wafer and from the back (or bottom) side of the wafer to attain the desired structure. Appropriate alignment schemes are used to effect this merger of front and back side processing. The back-side processing could be accomplished with other techniques, such as chemical etching, and, if desired, the fabrication could be carried out through the use of a silicon-on-insulator material rather than a single crystal wafer substrate. Such alternative approaches may be advantageous under certain circumstances.
Another aspect of the invention is the integration of large area millimeter-scale movable structures, micromechanical devices, and conventional microelectronic circuits on a common wafer, or substrate. The preferred plasma micromachining process utilized in the present invention facilitates this integration inasmuch as that process can be carried out on a substrate containing previously fabricated microelectronic devices.
A still further aspect of the invention is the provision of a large, relatively high mass element for use in an inertial sensor, or accelerometer, through the release of a large area solid block having substantially the full thickness of the substrate, the block being supported for relative motion in the substrate and being integral with micromechanical motion transducer elements fabricated from the substrate and located to detect and measure the motion of the block and hence the acceleration being applied to it, or which produce motion in the block, as by the application of electrostatic forces.
Still another aspect of the invention is the creation of a large, flat-surfaced platform within a wafer and mounted for motion in one or two dimensions through the integration of a micromechanical suspension and capable of use as a microstage. The large area microstage and its suspension are formed from an original flat substrate or wafer, the suspended structure being capable of moving in one or two directions in the plane of the structure. Such a microstage can be utilized in a data storage application by providing a mechanism for placing data on the surface, such as by formation of topological features, and a mechanism for sensing such data, such as by scanned probe devices.
In accordance with the present invention, it has been found that a large area, high mass movable structure can be fabricated on a wafer by combining deep reactive ion etch processes based on SF6 gas chemistries, which create single mask MEMS structures 20-50 xcexcm deep within a wafer or other substrate, with a process for etching through the entire thickness of the wafer. The resulting large area, solid structure can have a depth equal to the thickness of the wafer and can have arbitrary shapes and sizes, for example from 200 xcexcm up to 6-10 mm on a side. The large area structure is At supported in the wafer by MEMS structures such as cantilevered beams or springs and can be connected to electrical circuitry carried on, or formed in, the wafer by conventional integrated circuit techniques.
The structures of the present invention preferably are fabricated from a silicon wafer or similar substrate which typically is 300-400 xcexcm thick. A pattern for the MEMS support structure is formed by a standard photolithography process on the top surface of the wafer, and the pattern is then etched into the wafer by a silicon etch, leaving silicon islands, or mesas, having the shape of the desired MEMS support structure and the shape of the large area movable platform, all surrounded by top surface trenches with vertical walls. The top surface etch depth is approximately 20-50 xcexcm, with the mesas having widths as small as 1-3 xcexcm. The trench walls are then coated with an oxide.
A second photolithography step forms a pattern for the large area structure on the bottom surface of the wafer, this pattern being accurately aligned with the pattern on the front surface using conventional pattern alignment techniques. The bottom pattern defines the location of a bottom surface trench which will surround the large area structure to release it from the wafer, but which does not include the pattern of the top surface MEMS structure. This bottom surface trench is etched into the bottom of the wafer to a depth which is within about 10-30 xcexcm of the bottoms of the top surface trenches previously formed, so that the large area structure is not released from the wafer.
Thereafter, the MEMS structure on the top surface of the wafer is completed. This is accomplished by removing the oxide from the floor of the top-surface trench and then etching the exposed silicon to deepen the top surface trenches sufficiently far to intersect the bottom-surface trenches to release the large area structure. This large area structure is retained in the substrate by the mesas formed during the first top surface etching step. Thereafter, the mesas of the MEMS support structure are undercut in a release process to create a cantilever beam support structure array which is connected between selected locations on the substrate and the large area structure. The cantilever beam array suspends the large area structure in a cavity formed within the wafer by the top and bottom trench etches which combine to extend completely through the wafer. A metal layer may be applied, as by sputter coating, to coat the top surfaces and the sidewalls of the support structure to provide electrical surfaces which may serve as capacitors for actuating the device, for sensing its motion, and/or for providing connections to electrical circuitry.
The foregoing is a basic process for fabricating a MEMSxe2x80x94supported large area structure. More complex processes are needed to achieve higher performance. For example, a four masking level process can be used to allow the silicon beams themselves to act as capacitor electrodes. In this case, a patterned metal interconnect may be provided on the beams to provide the desired connections.
The large area portion of the wafer releasable by the above-described process can also be fabricated to have a reduced thickness in order to reduce its mass from the maximum provided by the full wafer thickness. Such a reduction in mass produces a platform-like structure that is more easily moved than the full-thickness block, while retaining the large surface area, and such a structure will be extremely useful in many applications; for example as a microstage. The reduced thickness structure can also be used as a data substrate for high density storage devices, for the large released area can serve as a platform upon which data bits are encoded. In this case a MEMS support structure can be used to scan the platform back and forth beneath a writing or reading head or probe with a positioning accuracy of 25 nm or better.
The large area structure can also be used as a optical deflector, with the MEMS actuators being used to move the reflective deflector surface.